The World Health Organization estimates that by 2020 major depressive disorder (MDD) will be the most common cause of disability worldwide. The medical and non-medical costs of MDD in the US exceed $80 billion annually. Although approximately 1 in 6Americans will suffer from MDD during their lifetime, roughly half of all patients with MDD go undiagnosed in primary care settings, and another fifth are incorrectly diagnosed as having MDD when they in fact have another psychiatric illness. Given the substantial medical, economic and social costs incurred when MDD is undiagnosed or untreated, there is tremendous need for a simple office- based biomarker test to aid clinicians in accurately identifying MDD. For a psychiatric diagnostic test to have clinical value, it must reliably distinguish not only between MDD and healthy individuals, but also between MDD and other significant Axis I disorders (e.g. bipolar disorder or anxiety disorders) that may present with symptoms that could be mistaken for MDD. No test currently exists that can accurately diagnose MDD and distinguish it from other psychiatric conditions. The potential market for an accurate diagnostic test for MDD is very large. Payers of medical services would cover the cost of such a diagnostic test because accurately identifying and treating MDD would reduce the high medical costs arising from medical service delivery to patients with untreated depression, as well as improving outcomes for patients with comorbid medical conditions such as diabetes or heart disease. Another market for an MDD diagnostic test are pharmaceutical companies. These companies have uniformly retreated from discovery efforts in mood disorders, in part due to the high rate of failed clinical trials, thought to arise in part from enrollment of inappropriae patients. An objective test of MDD could address this concern by providing certainty about the appropriateness of recruited patients, thereby enhancing confidence that trial results accurately reflect the true efficacy (or inefficacy) of investigated compounds. The Pax Neuroscience diagnostic biomarker Gsa Sequestration Assay (GSA), measures the localization of Gs, a crucial G protein that activates adenylyl cyclase to produce cyclic adenosine monophosphate (cAMP) and initiate downstream events. Gs shuttles between a state of sequestration in lipid rafts (relatively inactive), and a non-lipid raft location where it is freer to activate adenylyl cyclase. The Pax Neuroscience GSA biomarker measures the ratio of Gs between those states. Our preliminary data indicate that MDD patients have a significantly greater proportion of Gs captured in lipid rafts compared to non-depressed controls, which is consistent with other research indicating disrupted function of Gs, adenylyl cyclase and cAMP in MDD patients. These findings suggest the Pax Neuroscience GSA biomarker can accurately identify patients suffering from MDD. The proposed study will test the hypothesis that the ratio of Gs in and out of lipid rafts is an accurate and specific biomarker for MDD diagnostic purposes.